Almost everyone talks about Calibre, how neat it is but I have no clue what I can do with it?
I have my Sony PRS-T1 device, buy most of my books from Kobo store (or Sony ereader store but it is less user friendly I find, and price is same or sometimes higher than Kobo store). Buy books on my computer or directly on my ereader. Also I borrow books from public library.
That's all. Do I miss something?
I installed Calibre on my Mac but now what?

then clearly you don't need Calibre. It can do nothing for you. The whole point of Calibre is to manage (rate, view, filter, side load, etc.) your vast library of eBooks stored on your computer. In broad strokes, it does nothing that the computer version of the Sony Reader computer software doesn't do, except for being more flexible on source and DRM.

If you manage your e library solely from your reader device and feel no need to insert a 3rd party into the mix, then Calibre holds nothing for you.

That being said, if you have a Sony PRS 300 or 350 (Or other side load only readers), or If you have multiple eReader devices, or an Android device that can recieve ebooks from a variety of sources, having Calibre acting as a sort of intermediary between various flavors is pretty useful to have.

Almost everyone talks about Calibre, how neat it is but I have no clue what I can do with it?
I have my Sony PRS-T1 device, buy most of my books from Kobo store (or Sony ereader store but it is less user friendly I find, and price is same or sometimes higher than Kobo store). Buy books on my computer or directly on my ereader. Also I borrow books from public library.
That's all. Do I miss something?
I installed Calibre on my Mac but now what?

Add all your books from your different sources to it and use as a catalog... Instead of the Sony or Kobo store. Amongst other things, I use it to decide what I'm going to put on my readers next: it has cover art, the summary of the book, tags, can be searched and can be sorted in multiple ways. It is my "backup" copy of each book as well, directly on my computer. If you only have a few hundred books, I guess you could live without it, but when you have thousands it's definitely a "must". It has saved me from buying duplicates several times as I buy from multiple stores.
And that's apart from all the modifications you can make to your books. Also,with certain "tools" from "Apprentice Alf" you can remove the DRM which allows you to read books across several devices regardless of format (ie books purchased from Amazon on your Sony). I could go on much longer but these are the things I personally use it most for...

Add all your books from your different sources to it and use as a catalog... Instead of the Sony or Kobo store. Amongst other things, I use it to decide what I'm going to put on my readers next: it has cover art, the summary of the book, tags, can be searched and can be sorted in multiple ways. It is my "backup" copy of each book as well, directly on my computer. If you only have a few hundred books, I guess you could live without it, but when you have thousands it's definitely a "must". It has saved me from buying duplicates several times as I buy from multiple stores.
And that's apart from all the modifications you can make to your books. Also,with certain "tools" from "Apprentice Alf" you can remove the DRM which allows you to read books across several devices regardless of format (ie books purchased from Amazon on your Sony). I could go on much longer but these are the things I personally use it most for...

Buying books from Amazon.....ok am all interested now Did not know I could do that. Other than that, you're right. I don't think I need it now since my book collection is very small. I only buy books from Sony or Kobo (really don't know where else to go?) but I almost feel to buy a Kindle just for the Amazon collection of books and prices.

Buying books from Amazon.....ok am all interested now Did not know I could do that. Other than that, you're right. I don't think I need it now since my book collection is very small. I only buy books from Sony or Kobo (really don't know where else to go?) but I almost feel to buy a Kindle just for the Amazon collection of books and prices.

Calibre becomes your Kindle in a way once you have added those "additions" from our good friend Alf.

I buy from Amazon all the time.

Though my wife has a Kindle, I use Kindle for PC as the "download to" option within Amazon. Once Kindle for PC has the purchased ebook, it is a simple matter to transfer the Amazon ebook into Calibre. Certain "actions" are performed in the background along that transfer route and I can then edit and the transfer to any of my Sony ereaders.

If you want to purchase from Amazon, then Calibre is one of the tools you really would be advised to have.

Caibre is a much bettercollection manager than the sony library, you will have a problem loading kindle books to a sony reader as they tend to be DRM locked and then cannot be changed into a different format (apprentice Alf may help).
You can also get unlocked books from ebay, baen books, gutenberg etc.
Calibre cannot convert books that have DRM on them this includes public library books, these however can be read on sony devices as they are in EPUB format